Burners for combusting liquid and/or gaseous fuels, especially for use in a gas turbine, are known, which on the one hand have a high stability during operation, and on the other hand have good characteristics with regard to NOx values.
Thus, the so-called EV burner became known from EP-A1-321809. The premix burner which is described there is a conical burner which comprises a plurality of shells, a so-called double-cone burner, for creating a closed swirled flow in the cone head, which flow, on account of the increasing swirl along the cone point, becomes unstable and changes into an annular swirled flow with backflow in the core. Fuels, such as gaseous fuels, are injected along the passages which are formed by the individual adjacent shells, —also referred to as air inlet slots, and are homogeneously intermixed with air, before combustion commences as a result of ignition at the stagnation point of the backflow or backflow bubble, which fulfills the function of a device-free flame retainer. Liquid fuels are preferably injected via a central nozzle at the burner head and then evaporate in the cone cavity. A further important development in the field of premix burners involves the so-called AEV burner, as is known for example from EP-A1-704 657. The proposed burner has a swirl generator on the head side, which uses the aerodynamic basic principles of the EV burner which is already described above, for example according to EP-A1-0 321 809. This swirl generator is arranged upstream of a mixing section, the construction of which is explained in more detail further below. In principle, however, the use of an axial or radial swirl generator is also possible. Furthermore, it is also possible to provide a swirl generator which comprises a cylindrical or virtually cylindrical tube in which air flows into the inside of the tube via similar longitudinal slots, as in the case of the EV swirl generator, wherein the desired swirl formation of the air is carried out by means of a conically extending inner body for maximizing the sought-after premixing with a fuel which is injected at a suitable point, wherein this inner body conically tapers in the flow direction, with which the requirements for an efficient swirled flow are also provided in this case. Both the embodiments for the swirl generation which are referred to here, and the said printed publications, are an integrating element of this description. The mixing section itself preferably comprises a tubular mixing element, —subsequently referred to as a mixer tube, which permits a perfect premixing of the fuel, or fuels, which is or are used. The flow from the swirl generator in this case is transferred smoothly into the mixer tube. This takes place as a result of a transition geometry which comprises transfer passages which form the head part of this mixer tube, and which, as already indicated, transfer the flow into the adjoining effective throughflow cross section of the mixer tube. This loss-free, per se, flow guiding between swirl generator and mixer tube first of all prevents the direct formation of a backflow zone at the outlet of the swirl generator. Initially, the swirl intensity in the swirl generator is selected via its geometry so that the breaking up of the vortex does not take place in the mixer tube but further downstream at the combustion chamber inlet, wherein the length of this mixer tube is dimensioned so that a satisfactory mixing quality for all fuel types results. If, for example, the swirl generator which is used is constructed according to the principle features of the double-cone burner, then the swirl intensity results from the design of the corresponding cone angle, of the air inlet slots, and their number. In the mixer tube itself, the axial velocity profile has a distinct maximum on the axis and consequently prevents flashbacks in this region. The axial velocity decreases towards the wall. In order to also prevent flashbacks in this region, provision is made for various measures. For example, for one thing the overall velocity level can be raised by using a mixer tube with a sufficiently small diameter. Another possibility is to increase the velocity only in the outer region of the mixer tube by a small portion of the combustion air flowing into the mixer tube via an annular gap or through film layer holes downstream of the transfer passages.
Provision is frequently made in such burners for a plurality of fuel injection nozzles which are arranged in groups in order to thus ensure a stable combustion in different load ranges, for example special pilot nozzles for the low load range. In this case, the flame position can shift considerably, depending upon the piloting, and in such a case thermo-acoustic fluctuations can also occur in transition sections as a result of periodic change of the flame front positions.
These thermo-acoustic oscillations constitute a danger for each type of combustion application. They lead to pressure oscillations of high amplitude, to limitation of the operating range, and can increase pollutant emissions. This especially applies to combustion systems with low acoustic damping, such as annular combustion chambers with reverberative walls. In order to enable a high power conversion over a wide operating range with regard to pulsations and pollutant emissions, an active control of the combustion oscillations may be necessary.
In the prior art, as combustion concepts for partial load operation of such burners, for example so-called burner staging is known, in which individual burners are purposefully deactivated so that the remaining burners can be operated at full load. In particular, in the case of annular combustion chambers with a plurality of burner rings of different radius, which are offset in relation to each other, this concept can be used quite successfully.
As a result of fuel staging inside a burner, the flame position can be influenced and therefore influencing of flow instabilities and also time delay effects can be reduced (described for example in EP-A1-1 292 795).
It is also known to provide so-called pilot lances in such burners. Pilot fuel (gaseous or liquid) can be specifically fed centrally via a lance for the piloted operation of the burner, as is described for example in EP-A1-0 778 445 for the case of a double-cone burner, and in WO-A-93/17279 and also in EP-A1-0 833 105 for premix burners without, or with, a downstream mixing section.